Method and apparatus for transmitting information

ABSTRACT

A method and apparatus for transmitting information comprises a signal generator (1) for continuously generating a pair of optical signals (f1, f2). These signals are similarly modulated in a modulator (5) with information to be transmitted and are combined with their polarizations substantially orthogonal. The combined signals are transmitted with the portions of each signal modulated with the same information being transmitted together down an optical fiber.

The invention relates to methods and apparatus for transmittinginformation.

It is well known that conventional coherent optical detection systemsonly give optimum performance when the signal and local oscillatorpolarisations are identical. For practical systems this represents aproblem because an optical signal propagating through a conventionalsingle-mode fibre undergoes random polarisation changes and this causesthe received polarisation state to be both indeterminate and timevarying. To date this problem has been overcome using a variety oftechniques including polarisation tracking receivers, polarisationdiversity receivers, and polarisation scrambling.

It is a known property of many transmission media, including opticalwaveguides such as optical fibres, that although random changes inabsolute polarisation may take place during transmission through themedium, these random variations are the same for each signal so that apair of signals which are initially orthogonally polarised will remainorthogonal.

An article entitled "A polarization-insensitive coherent lightwavesystem using wide-deviation FSK and data-induced polarization switching"by L. J. Cimini et al, Electronic Letters Vol 23 December 1987 pp1365-1366 discloses a polarisation-insensitive technique takingadvantage of the stable orthogonal relationship of two polarisationstates in which the frequency shift in an FSK signal inducespolarisation switching by introducing a passive device with highbirefringence in the path of the transmitted signal. Thus, as the signalswitches from one frequency to another, its polarisation switchesbetween mutually orthogonal polarisation states. However such atechnique is useful only with FSK modulation.

In accordance with one aspect of the present invention, a method oftransmitting information comprises launching a pair of orthogonalpolarised optical signals having different frequencies, each of which isidentically modulated with the information to be transmitted, into anoptical waveguide which causes the signals to undergo randompolarisation changes during propagation through the waveguide anddetecting said signals after propagation through the waveguide by meansof a coherent optical receiver.

Thus, at the receiver, irrespective of the polarisation of the localoscillator signal, two electrical interference signals will be generatedafter the optical mixing process with their amplitudes varying inantiphase only one of which can be zero at a given time.

In accordance with a second aspect of the present invention, an opticalnetwork comprises signal generating means for generating a pair oforthogonally polarised signals having different frequencies and beingmodulated in accordance with the information to be transmitted; andtransmitting means for transmitting the modulated signals with theportions of each signal modulated with the same information beingtransmitted together.

The invention may be used with a wide variety of different types ofsignal but is particularly suitable for use with signals havingfrequencies within the optical band. In that case, the transmissionmedium will comprise an optical waveguide such as an optical fibre.

In one example, the signal generating means comprises carrier signalgenerating means for generating signals with two frequencies, combiningmeans for combining the two signal with the polarisation of one signalorthogonal to the other, and modulating means for modulating the signalsin accordance with the information to be transmitted.

The modulating means may be positioned downstream of the combining meansalthough it would also be possible to provide modulating means formodulating each of the two signals separately, the signals subsequentlybeing combined.

In a second example, the signal generating means comprises carriersignal generating means for generating two carrier signals withdifferent frequencies, signal coupling means for coupling the twosignals, the coupled signals being fed to the modulating means, and abirefringent device downstream of the modulating means for imparting theorthogonal polarisations on the two modulated carrier signals inresponse to their frequencies. Alternatively the carrier signals couldbe individually modulated prior to being coupled while the birefringentdevice could be upstream of the modulating means.

In a third example, the signal generating means comprises a carriersignal generating means for generating a single carrier signal;modulating means for modulating the carrier signal in accordance withthe information to be transmitted; and means for generating the twoorthogonally polarised and frequency shifted versions of the carriersignal.

The means for generating the orthogonally polarised signals may comprisea signal splitter for generating two versions of the modulated carriersignal, frequency shifting means for shifting the frequency of one ofthe two versions, polarisation control means for adjusting thepolarisation of the signals to be orthogonal to each other, andcombining means for combining the orthogonally polarised versions of themodulated carrier signal.

In one arrangement the polarisation control means could be provided bythe signal splitter which in the case of an incoming linear signal with45° polarisation would split them into two orthogonally polarisedsignals.

Typically, the modulating means will be provided separately from themeans for generating the carrier signal or signals but in some cases,the modulating means could be formed by part of the carrier signalgenerating means so that the signal generated is already modulated.Thus, in the case of a laser, for example, this could be directlymodulated by the information.

The invention is also suitable for transmitting frequency multiplexedoptical signals. Thus, in accordance with a third aspect of the presentinvention, apparatus for transmitting a signal comprising a number offrequency multiplexed channels comprises means for generating a secondversion of the multiplex signal having frequencies different from thoseof the multiplex signal; and means for transmitting the two versions ofthe signal with orthogonal polarisations and with the portions of eachsignal corresponding to the same channel being transmitted together.

In accordance with a fourth aspect of the present invention, a method oftransmitting a signal comprising a number of frequency multiplexedchannels comprises generating a second version of the multiplex signalhaving frequencies different from those of the multiplex signal; andtransmitting the two versions of the signal with orthogonalpolarisations and with the portions of each signal corresponding to thesame channel being transmitted together.

Examples of methods and apparatus in accordance with the presentinvention will now be described with reference to the accompanyingdrawings, in which:-

FIGS. 1a-1c illustrate signals modulated in accordance with ASK, FSK,and PSK methods respectively;

FIG. 2 illustrates a first example of a transmitter;

FIG. 3 illustrates a second example of a transmitter;

FIGS. 4a and 4b illustrate two forms of signal source for use in theFIG. 2 example;

FIG. 5 illustrates a third example of a transmitter;

FIG. 6 illustrates a first example of a receiver for receiving an ASKmodulated signal;

FIG. 7 illustrates a second form of receiver;

FIGS. 8a and 8b illustrate a frequency mulitplexed signal before andafter processing in accordance with the invention respectively;

FIG. 9 illustrates a generalised form of transmitter; and,

FIGS. 10a and 10b illustrate examples of apparatus for generating twocarrier signals.

If two orthogonal optical carriers with different frequencies aremodulated and transmitted through a birefringent medium the receivedoptical field (e_(s)), can be expressed generally as

    e.sub.S =E.sub.S [√(K.sub.SX) Cos (ω.sub.Sl t+m.sub.1(t))+j√(K.sub.SY) Cos (ω.sub.Sl t+δ.sub.S +m.sub.1(t))]m.sub.2(t)

    +E.sub.S [√K.sub.SY Cos (ω.sub.S2 t+m.sub.1(t))-j√(K.sub.SX) Cos (ω.sub.S2 t+δ.sub.S +m.sub.1(t))]m.sub.3(t)                                   (1)

Complex notation is used to indicate orthogonal field components,ω_(S1), ω_(S2) are the two optical carrier frequencies, K_(SX), K_(SY)represent the fraction of the received optical power in orthogonal X andY planes and δ_(S) is the phase relationship between the two signals.m₁(t), m₂(t) and m₃(t) are modulation parameters; m₁(t) represents phaseor frequency modulation and the other two represent amplitude. Thisnotation is described in more detail in HODGKINSON T.G.: Receiveranalysis for synchronous coherent optical fibre transmission systems,IEEE. j. Lightwave Technol. 1987, LT-5, pp. 573-586.

e_(S) can be detected using coherent (homodyne or heterodyne) techniquesor e_(S) ² can be detected using direct detection for certain types ofmodulation such as ASK. Heterodyne detection will be described in detailbut the results can be taken as being generally applicable to the othertypes of detection. The normalised input to the heterodyne receiver'sbaseband filter (v_(I/P)), when using non-synchronous (square-law) IFdemodulation, is

    v.sub.I/P =(K.sub.P1 +K.sub.P2)K.sub.m f.sub.(t)           (2)

and for synchronous (linear) IF demodulation it is

    v.sub.I/P =(√K.sub.P1 +√K.sub.P2)K.sub.m f.sub.(t)(3)

where

    K.sub.P1 =K.sub.SX K.sub.LX +K.sub.SY K.sub.LY +2√(K.sub.SX K.sub.LX K.sub.SY K.sub.LY) Cos (δ.sub.L -δ.sub.S)     (4)

    K.sub.P2 =K.sub.SY K.sub.LX +K.sub.SX K.sub.LY -2√(K.sub.SX K.sub.LX K.sub.SY K.sub.LY) Cos (δ.sub.L -δ.sub.S)     (5)

K_(LX), K_(LY) and δ_(L) are local oscillator polarisation parameterssimilar to K_(SX), K_(SY), δ_(S) ; K_(m) is determined by the modulationformat and f.sub.(t) represents the digital modulation and takes thevalue 1 or 0.

Substituting equations (4) and (5) into (1) and (2) shows that for allpossible signal/local osillator polarisation combinations v_(I/P) isconstant for non-synchronous IF demodulation and varies by √2 forsynchronous demodulation; when at a minimum this equals thenon-synchronous v_(I/P) value. However, irrespective of which type of IFdemodulation is used the worst case performance should never be morethan 3 dB worse than standard polarisation aligned heterodyne detection.

Some examples of possible waveforms e_(S) are given in FIG. 1. In eachcase the binary digits 1 0 1 are being transmitted. In FIG. 1a, ASKmodulation is used, the two different and orthogonally polarisedfrequencies being denoted f₁, f₂. Both frequencies are attenuated tozero for a binary zero.

In FIG. 1b, an FSK system is shown in which a binary 1 is represented bythe orthogonally polarised frequencies f₁, f₂ while a binary zero isrepresented by orthogonally polarised frequencies f₃, f₄ (different fromf₁ and f₂).

FIG. 1c illustrates a PSK system with the two binary data values beingrepresented by the frequencies f₁, f₂ both of which are shifted in phaseby 180° for the two values.

The first example of a transmitter is shown in FIG. 2 and comprises anoptical source 1 to be described in more detail below which generatestwo coherent carrier signals with the frequencies f₁, f₂. These signalsare fed to respective polarisation controllers 2, 3 which adjust thesignals to become orthogonally polarised. These polarised signals arefed to a polarisation selective coupler 4 which combines the signalstogether and feeds the combined signals to a modulator 5. The signalsare then modulated by the modulator 5 in accordance with the data to betransmitted, the resultant signal taking one of the forms shown in FIGS.1a to 1c depending upon the form of the modulation.

A second transmitter is shown in FIG. 3 in which the optical source 1'generates a single carrier signal with the frequency f₁ which is fed toa modulator 6. Once again, the modulator 6 imparts the requiredmodulation onto the carrier and the resultant modulated carrier is fedto an optical splitter 7 which generates two versions of the modulatedsignal. The first version is fed to a polarisation controller 8 whilethe second version is fed via a frequency shifting circuit 9 to apolarisation controller 10. The frequency shifter 9 adjusts thefrequency of the incoming signal so that the signal output from thefrequency shifting circuit 9 has a carrier frequency f₂ and thepolarisation of this signal is then adjusted by the controller 10 to beorthogonal to the polarisation of the signal f₁ from the controller 8.The orthogonally polarised signals are then combined by a polarisationselective coupler 11 for transmission. It will be understood that asuitable delay will be provided in the path to the controller 8 tocompensate for any delays in the frequency shifter 9.

In some cases, the modulator 6 can be dispensed with and the opticalsource 1 could be controlled directly by the data to generate amodulated output signal as indicated by the dashed line in FIG. 3.

FIG. 4a illustrates an example of the optical source 1 of FIG. 2. Inthis case, two sources 12, 13, such as lasers, are provided forgenerating the carrier signals f₁, f₂ respectively. The signal f₁ issampled by a splitter 14 and fed back via a control circuit 15 to theoptical source 13 so as to ensure that the frequency f₂ is kept constantrelative to the frequency f₁.

An example of the construction of the optical source 1' of FIG. 3 isshown in FIG. 4b. In this case, a single laser 16 generates a signal f₁which is fed to a splitter 17 which generates two versions of thesignal. One version is fed out of the source 1' as the carrier signal f₁while the other signal is fed to a frequency shifting circuit 18 whichadjusts the incoming frequency to f₂.

FIG. 5 illustrates a third transmitter configuration in whichunmodulated, carrier signals with frequencies f₁, f₂ are combined in anoptical coupler 27, the combined signal being fed to a modulator 28controlled by the data signal. The modulated signal is then fed to a(passive or active) birefringent device or medium which has acharacteristic such that the incoming carrier frequencies f₁, f₂ arecaused to become orthogonally polarised.

FIG. 9 illustrates a generalised form of transmitter for which FIG. 5 isa particular example. In the FIG. 9 example, a laser 1 generates anoptical signal at a particular frequency which is fed to a device 50 forgenerating from the incoming signal A two signals having differentfrequencies B. These signals are then fed to a birefringent device 51which rotates the polarisations of the two frequencies so that they aremutually orthogonal. In order to modulate data onto the two polarisedsignals, the data can be used to modulate the laser 1 directly or can beused to control a modulator 52 positioned at any one of the threepositions shown in FIG. 9.

FIG. 10 illustrates an example of a modulator for use in this invention,such as the modulator 52. It is well known that when two electricalsinusoidal signals (in general one only need be sinusoidal so the othercould be a modulated carrier) are multipled the resulting outputspectrum consists of sum and difference frequencies only.

    2 Cos (ω.sub.1 t) Cos (ω.sub.2 t)=Cos ((ω.sub.1 -ω.sub.2)t)+Cos ((ω.sub.1 +ω.sub.2)t)

This is commonly referred to as double sideband suppressed carrier(DSB-SC).

It follows from the above that if a device existed which effectivelymultiplied an electrical signal and an optical signal, two opticalsignals, separated by twice the frequency of the electrical signal,could be generated from a single optical source (FIG. 10a). In practicethis multiplying function may have to be approximated using, forexample, the arrangement shown in FIG. 10b. The reason why thistechnique is approximate is that the harmonics of the electrical squarewave also produce optical frequency components, but with correct systemdesign this would only appear as a loss of signal power. This problemcannot be overcome by using a sinusoidal electrical signal because thisgenerates additional optical frequency components according to theBessel functions.

A simple form of receiver configuration for demodulating an ASKmodulated signal of the form shown in FIG. 1a is illustrated in FIG. 6.The incoming signal is fed into one input arm of an optical coupler 30and a local oscillator frequency is fed into the other input arm of thecoupler 30. This local oscillator signal comes from a local oscillator31. The two output signals from the coupler 30, which correspond tomixed versions of the two orthogonally polarised incoming signals withthe local oscillator signal, are fed to an optical receiver 32 such asphotodetector and associated electronics which generates a correspondingoutput electrical signal which is fed to an IF filter 33. The IF filterbandwidth will typically be twice the bit-rate in Hertz+f₂ -f₁. A filter33 is shown as a dashed box because it may not be essential when usingsynchronous IF demodulation. The local oscillator frequency iscontrolled by a control circuit 34 which monitors the frequency of themixed signal received by the receiver 32.

The IF signal is fed to an IF demodulator circuit 35 which may operateon a square law principle. Since the incoming signal has been ASKmodulated, the result of squaring a mixed IF frequency will yield a zeroamplitude signal (binary 0) or a fixed, positive amplitude signal(binary 1) which is fed to a base band filter 36.

It should be noted that since the incoming, modulated signal comprisestwo orthogonally polarised components, there will always be at least onesignal resulting from the mixing of the (non-zero amplitude) modulatedsignal with the local oscillating frequency and the problem of fadingwill not arise.

FIG. 7 illustrates a more generalised receiver configuration which canbe used to demodulate any of the signal types shown in FIG. 1. Incontrast to the FIG. 6 example in which the bandwidth of the IF filter33 must be sufficient to include both mixed signals, in the FIG. 7example, the mixed signals are split and passed through separate IFfilters 37, 38. For the waveforms of FIGS. 1a and 1c, these will be setin a similar way to a standard FSK receiver with each filter passing arespective signal corresponding to the mixing of either f₁ or f₂ withthe local oscillator frequency. In the case of the FIG. 1b signal, theIF filter bandwidths must be increased and values equivalent of thosefor the receiver configuration in FIG. 6 are needed. The filteredsignals are demodulated in respective demodulator circuits 39, 40 whichmay operate on a square law principle and the demodulated signals arethen fed to a combining circuit 41 which effectively inverts and addsone of the signals to the other, the resultant signal being fed to thebaseband filter 36.

In a still more generalised form, each arm of FIG. 7 (comprising afilter and demodulator) could be doubled up to provide one arm for eachIF.

It is also possible to use the invention to transmit a frequencymultiplex. The advantage of this is that the component needed to producethe orthogonal optical carriers is not required at every transmitter sothat one per transmission waveguide is all that is needed. Afterassembling the frequency multiplex with the appropriate frequencyseparations (FIG. 8a), the multiplex signal is processed using thedevice shown in FIG. 3 with the multiplex signal being supplied to thesplitter 7 in place of the signal from the optical source 1' andmodulator 6. The output signal from this configuration will have theform shown in FIG. 8b. The pair of frequencies associated with eachchannel will be orthogonally polarised.

It should be understood that in all the examples described, the twooptical carrier frequencies will typically be separated by two to threetimes the bit rate in hertz.

We claim:
 1. A method of transmitting information comprising launching apair of orthogonal polarised optical signals having differentfrequencies, each of which is identically modulated with information tobe transmitted, into an optical waveguide which can cause the signals toundergo random polarisation changes during propagation through thewaveguide and detecting said signals after propagation through thewaveguide by means of a coherent optical receiver.
 2. A method asclaimed in claim 1 including the step of generating a pair of opticalsignals having different frequencies and orthogonal polarisations andmodulating the signals identically with the information.
 3. A method asclaimed in claim 2 in which the optical signals are modulated afterbeing combined.
 4. A method as claimed in claim 1 including the step ofgenerating a pair of optical carrier signals of different frequencies,combining the optical signals, modulating the combined signals, andfeeding the modulated combined signals to a birefringent device whichimparts the orthogonal polarisations on the two modulated opticalcarrier signals in response to their different frequencies.
 5. A methodas claimed in claim 1 including the steps of generating a single opticalcarrier signal, modulating the carrier signal in accordance with theinformation to be transmitted, and generating the pair of orthogonallypolarised optical carrier signals of different frequencies.
 6. A methodas claimed in claim 5 in which the orthogonally polarised opticalsignals are generated by taking the modulated, single optical carriersignal, generating two versions of this signal, shifting the frequencyof one version relative to the other, adjusting the polarisation of thesignals to be orthogonal to each other and combining the orthogonallypolarised versions of the modulated carrier signal.
 7. A method oftransmitting a signal comprising a number of frequency multiplexedchannels, the method comprising generating a second version of themultiplex signal having frequencies different from those of themultiplex signals; and transmitting the two versions of the signal withorthogonal polarisations and with the portions of each signalcorresponding to the same channel being transmitted simultaneously intoan optical waveguide which can cause the signals to undergo randompolarisation changes during propagation through the waveguide; anddetecting said signals after propagation through the waveguide by meansof a coherent optical receiver.
 8. An optical network comprising anoptical waveguide which can cause optical signals to undergo randompolarisation changes during propagation through the waveguide,generating means for generating a pair of orthogonally polarised signalshaving different frequencies, each of which is identically modulated inaccordance with information to be transmitted, and means for launchingthe signals simultaneously into the optical waveguide and a coherentdetector for detecting said signals.
 9. A network according to claim 8,wherein the signal generating means comprises optical carrier signalgenerating means for generating signals with two frequencies, combiningmeans for combining the two signals with the polarisation of one signalorthogonal to the other, and modulating means for modulating the signalsidentically in accordance with the information to be transmitted.
 10. Anetwork according to claim 9, wherein the modulating means is positioneddownstream of the combining means.
 11. A network according to claim 8,wherein the signal generating means comprises carrier signal generatingmeans for generating two carrier signals with different frequencies,signal coupling means for coupling the two signals, the coupled signalsbeing fed to the modulating means, and a birefringent device downstreamof the modulating means for imparting the orthogonal polarisations onthe two modulated carrier signals in response to their frequencies. 12.A network according to claim 8, wherein the signal generating meanscomprises a carrier signal generating means for generating a singlecarrier signal; modulating means for modulating the carrier inaccordance with the information to be transmitted; and means forgenerating the two orthogonally polarised and frequency shifted versionsof the carrier signal.
 13. A network according to claim 12, wherein themeans for generating the orthogonally polarised signals comprises asignal splitter for generating two versions of the modulated carriersignal, frequency shifting means for shifting the frequency of one ofthe two versions, polarisation control means for adjusting thepolarisation of the signals to be orthogonal to each other, andcombining means for combining the orthogonally polarised versions of themodulated carrier signal.
 14. A network according to claim 8, whereinthe modulating means is separate from the means for generating thecarrier signal or signals.
 15. A network according to claim 8, whereinthe modulating means modulates the carrier signal or signals inaccordance with an ASK, FSK, or PSK system.
 16. A network fortransmitting a signal comprising a number of frequency multiplexedchannels comprises means for generating a second version of themultiplex signal having frequencies different from those of themultiplex signal, and means for transmitting the two versions of thesignal with orthogonal polarisations and with the portions of eachsignal corresponding to the same channel being transmittedsimultaneously into an optical waveguide which can cause randompolarisation changes during propagation through the waveguide and acoherent optical detector for detecting said signals.